Etch and polish for metal halides

ABSTRACT

Boric acid, an acidifying agent, and an optional complexing agent are mixed to form an aqueous solution which is used to etch and polish metal halide. The solution is used to wet a polishing pad which is mounted on a rotating planar wheel. The metal halide is brought into contact with the pad thereby polishing the surface of the metal halide.

BACKGROUND OF THE INVENTION

This invention relates generally to etching and polishing compositionsand particularly to a etch and polish composition for metal halides.

Metal halides, especially single crystal alkaline earth metal halides,have applications as semiconductor substrates for devices such asphotovoltaic diodes, lasers, transistors, and charge coupled devices.For example, U.S. Pat. No. 4,080,723 discloses using single crystalbarium fluoride as a substrate for group IV-VI semiconductor diodes toprovide detectors for infrared radiation in thermal imaging systems.When employing metal halide salts, such as single crystal bariumfluoride as a substrate for such semiconductor applications, it becomesnecessary to etch, polish, or otherwise ablate a portion of the salt inorder to insure a smooth substrate surface. Prior methods for obtaininga smooth substrate surface have included both mechanical and chemicaltechniques. U.S. Pat. No. 3,935,302 discloses a mechanical method forobtaining a smooth substrate surface which uses diamond impregnatedblades. Recently the usefulness of chemical solutions to removeirregularities on crystal surfaces has been demonstrated by severalinvestigators. Polishes have been developed for such materials as NaCland KCl to produce high quality substrates for laser applications. U.S.Pat. No. 4,040,896 discloses a chemical polish for BaF₂ and CaF₂ whichuses a mixed sulfuric acid and acetic acid solution for a predeterminedimmersion period not exceeding about 16 minutes per cycle for CaF₂ butcan be extended for several hours per cycle for BaF₂. The preferredsulfuric to acetic acid parts ratio is about 4 to 9 for polishing theBaF₂ crystal surface and about 3 parts to 2 parts for polishing the CaF₂crystal surface. U.S. Pat. No. 4,155,803 discloses the use of monovalentacids, including especially inorganic acids such as nitric acid, as wellas halogen containing acids, as preferred hydrochloric acid, tochemically ablate the metal halide salt in aqueous solutions whenexposed to low temperatures thereby producing a smooth surface.

Use of mechanical methods, however, to smooth the substrate surface cancause undesired chipping and cracking that impairs the usefulness of thedevice. Similarly, prior chemical methods have not obtained the requiredsmoothness of the crystal surface which also limits the usefulness ofthe device.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide aprocess for etching and polishing the surface of metal halide crystals.

Another object of this invention is to provide a chemical compositionfor etching and polishing the surface of metal halide crystals.

Another object of this invention is to provide a metal halide crystalhaving a very smooth surface suitable for use as a substrate.

These and other objects are accomplished by exposing the metal halide toa boric acid solution containing an acidifying agent and optionally acomplexing agent. In the preferred embodiment, boric acid, a complexingagent, and glacial acetic acid are mixed to form an aqueous solutionwhich is used to polish the metal halide. The solution is used to wet apolishing pad which is mounted on a rotating planar wheel. The metalhalide is brought into contact with the pad thereby polishing thesurface of the metal halide.

Other objects, advantages, and novel features of the present inventionwill become apparent form the following detailed description of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

An etchant and polish for slightly soluble metal halides comprises asolution of boric acid (H₃ BO₃) containing an acidifying agent. Adesirable, but not necesary refinement of the invention, is theinclusion of a complexing agent to aid and regulate the dissolution ofthe metal halide, particularly metal fluorides. The boric acid promotessolution of the metal halides by formation of haloborates in thepresence of hydrogen ions, i.e. for BaF₂, Ba(BF₄)₂ is formed. The rateof dissolution, etching or polishing, is determined by many experimentalvariables, some of which are: concentration of boric acid and acidifyingagent, temperature, and orientation of the crystal since not allcrystalline faces are attacked equally. The presence and concentrationof a complexing agent further regulates the rate and specificity of thedissolution process. When the dissolution is conducted in an unstirredsolution etch pits are formed; when the solution is used with apolishing pad a smooth damage free surface results from the increasedsolubility of the fluoride, if proper crystalline orientations areselected.

The preferred metal halides are BaF₂, CaF₂, ZrF₄, and LaF₃, withBaF.sub. 2 being most preferred. Barium fluoride is a particularlydifficult material to etch or polish because of its very slightsolubility.

In general, weak organic acids with dissociation constants between1×10⁻² and 1×10⁻⁶ work very well in the present invention. Acetic,propionic, citric, oxalic and malonic acids are preferred with aceticacid being most preferred. Any complexing agent, either in the acid formor as one or more of several possible soluble salts, could function inthe present invention but nitrilotriacetactic acid (NTA),ethylenediamine tetraacetic acid (EDTA), citric acid, and tartaric acidare preferred with nitrilotriacetactic acid (NTA) being most preferred.

The solution of the present generally comprises between about 1-30percent by weight of boric acid, between about 0.1-20 percent by weightof an acidfying agent, and up to about 15 percent by weight of acomplexing agent.

Generally, the solution is placed in contact with the metal halidecrystal and allowed to stand until the etch pits are developed acrossthe crystal surface corresponding to crystalline defects. Alternatively,the solution is used to wet a polishing pad which is mounted on a devicewhich can rub the pad against the halide crystal, typically a rotatingsurface. The solution on the pad is contacted with the halide crystalthereby producing a uniformly polished crystal surface.

The invention having been generally described, the following examplesare given as particular embodiments of the invention and to demonstratethe practice and advantages thereof. It is understood that the examplesare given by way of illustration and are not inteneded to limit thespecification or the claims to follow in any manner.

EXAMPLE I

A synthetic nonwoven polishing pad mounted on a rotating planar wheeland wetted with a solution comprising the proportions 20 ml H₃ BO₃solution (4% by weight), 0.7 gm NTA (trisodium salt), and 2 ml glacialacetic acid in a solution volume of 50 ml produced a uniformly polishedBaF₂ surface when contacted with a crystal.

EXAMPLE II

A synthetic nonwoven polishing pad mounted on a rotating planar wheeland wetted with a solution comprising the proportions 20 ml H₃ BO₃solution (4% by weight) and 3 gm tartaic acid in 100 ml volume produceda uniformly polished BaF₂ surface when contacted with a crystal.

EXAMPLE III

A synthetic nonwoven polishing pad mounted on a rotating planar wheeland wetted with a solution comprising the proportions 20 ml (4% byweight) H₃ BO₃ and 4.4 gm of citric acid in a solution volume of 50 mlproduced a uniformly polished CaF₂ surface when contacted with acrystal.

EXAMPLE IV

BaF₂ was immersed in a solution of 4% boric acid containing 1.7 moles ofacetic acid per liter. After standing two hours at 45° C. etch pits aredeveloped across the crystal surface corresponding to crystallinedefects.

Etches and polishes are useful for delineating defects in crystallinematerial and in producing a smooth, damage free surface suitable forsubsequent use, e.g., as a substrate for epitaxial growth. BaF₂ andsimilar materials are useful as substrates and in a variety ofapplications requiring infrared transparent materials.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A composition suitable for etching and polishingmetal halides in an aqueous solution, comprising: between about 1-30percent by weight of boric acid;between about 0.1-20 percent by weightof an acidfying agent; and p1 up to about 15 percent by weight of acomplexing agent.
 2. The composition of claim 1 wherein said acidfyingagent is a weak organic acid with a dissociation constant between 1×10⁻²and 1×10⁻⁶.
 3. The composition of claim 2 wherein said acidfying agentis selected from the group consisting of acetic, propionic, citric,oxalic and malonic acids.
 4. The composition of claim 3 wherein saidacidfying agent is acetic acid.
 5. The composition of claim 1 whereinsaid complexing agent is is selected from the group consisting ofnitrilotriacetactic acid (NTA), ethylenediamine tetraacetic acid (EDTA),citric acid, and tartaric, acid, either as acids or as salts thereof. 6.The composition of claim 5 wherein said complexing agent isnitrilotriacetactic acid (NTA), either as the acid or salt thereof.
 7. Aprocess for etching and polishing metal halides in an aqueous solution,comprising the steps of;contacting the aqueous solution with said metalhalide, said aqueous solution comprising:between about 1-30 percent byweight of boric acid; between about 0.1-20 percent by weight of anacidifying agent; and up to about 15 percent by weight of a complexingagent.
 8. The process of claim 7 wherein said contacting step furthercomprises the steps of:wetting a polishing cloth with said solution;contacting said polishing cloth with said metal halide; and moving saidcloth with respect to said metal halide thereby polishing said metalhalide.
 9. The process of claim 8 wherein said metal halide is selectedfrom the group consisting of BaF₂, CaF₂, ZrF₄, and LaF₃.
 10. The processof claim 9 wherein said contacting step comprises contacting saidsolution with BaF₂.
 11. The product of the process of claim
 7. 12. Theproduct of the process of claim
 10. 13. A process for etching andpolishing metal halides, in an aqueous solution comprising the stepsof;contacting the aqueous solution with said metal halide, said aqueoussolution, comprising:about 3 percent by weight of boric acid; about 6percent by weight of an acidifying agent; and about 2 percent by weightof a complexing agent.
 14. The process of claim 13 wherein saidcontacting step further comprises the steps of:wetting a polishing clothwith said solution; contacting said polishing cloth with said metalhalide; and moving said cloth with respect to said metal halide therebypolishing said metal halide.
 15. The process of claim 14 wherein saidmetal halide is selected from the group consisting of BaF₂, CaF₂, ZrF₄,and LaF₃.
 16. The process of claim 15 wherein said contacting stepcomprises contacting said solution with BaF₂.
 17. The product of theprocess of claim 13
 18. The product of the process of claim 16.